ABSTRACT Deaths from opioid overdose continue to rise; from 2015-2016, there was a 28% increase in the number of fatal overdoses. Fentanyl derivatives are inexpensive, easy to synthesize, potent, and marketed to unsuspecting abusers as heroin or other drugs. Moreover, the effects of fentanyl derivatives are reportedly more difficult to reverse with naloxone, compared with reversal of heroin. Pharmacotherapies for opioid abuse include the opioid receptor agonists methadone and buprenorphine that are effective in many patients, although both drugs have limitations, including diversion and abuse, and they can have serious unwanted effects, including respiratory depression and death. The opioid receptor antagonists naltrexone and naloxone avoid the abuse liability and adverse effects of methadone and buprenorphine; however, short durations of action and surmountability limit their effectiveness. A medication with a longer duration of action that prevents and reverses the effects of opioids in a manner that is not surmounted by increasing doses of agonist could improve significantly treatment of abuse and save lives by providing insurmountable extended protection after rescue from overdose. Our pilot studies in monkeys show that the pseudoirreversible, opioid receptor selective antagonist methocinnamox (MCAM) decreases heroin but not cocaine self-administration, decreases choice for remifentanil in a food/drug choice procedure, and reverses as well as protects against respiratory depression by heroin, with a single injection being effective for a week or longer. Proposed studies build on these compelling data and examine the long-term antagonist properties and the pharmacokinetics of MCAM in combination with commonly abuse opioids, including ultra-potent fentanyl analogs. MCAM is hypothesized to be better than naloxone and naltrexone in reversing and preventing the effects of opioid receptor agonists and, in particular, high efficacy agonists that exert behavioral effects when occupying relatively few opioid receptors. Its pseudoirreversible binding is expected to make antagonism by MCAM more difficult to surmount and to provide longer antagonist action than the currently used opioid receptor antagonists. Aim 1 will characterize long-term antagonism of heroin self-administration by MCAM in a food/drug choice procedure. Aim 2 will examine the ability of MCAM to antagonize the positive reinforcing and respiratory depressant effects of fentanyl and ultra- potent analogs alone and in mixtures with heroin or cocaine. Aim 3 will characterize the pharmacokinetic profile of MCAM, heroin, fentanyl and its derivatives, and cocaine, alone and in mixtures. Using a highly translatable species, this project will examine a novel opioid receptor antagonist that has the potential to save lives by preventing and reversing the adverse, and often lethal, effects of opioids. The availability of another safe, effective, and long-acting treatment could be advantageous for many patients (e.g., problems with compliance would be reduced by an extended-release, pseudoirreversible antagonist) and in many treatment settings (e.g., rural areas where the opioid epidemic is worsening and regular contact with treatment providers is not practical).